1. Field of the Invention
The present invention relates to an image forming apparatus using an electrophotographic method, and a control method. The present invention is suitable for an image forming apparatus such as a copying machine, printer, facsimile apparatus, and the like.
2. Description of the Related Art
Along with prevalence of image forming apparatuses such as laser printers and the like, such image forming apparatuses are increasingly required to attain higher image quality and to reduce cost. An image forming apparatus includes a primary charger for uniformly charging a photosensitive member, a primary transfer unit for transferring a toner image formed on the photosensitive member onto an intermediate transfer belt, and a secondary transfer unit for transferring the toner image on the intermediate transfer belt on a print sheet.
As components of the primary and secondary transfer units, in recent years, a transfer member of a contact transfer type (contact transfer member) represented by a transfer roller becomes mainstream. The contact transfer member can realize a size reduction of a power supply capacity and a reduction of the generation amount of discharge products (ozone and the like) compared to a corona charger of a non-contact type and the like. The transfer roller includes, for example, a shaft, and an elastic layer of a middle resistance, which is formed around the shaft, and is brought into pressure contact with the intermediate transfer belt or print sheet at a predetermined pressure to form a transfer part (transfer nip). While a toner image is passing through the transfer part (i.e., during an interval from when the toner image reaches the transfer part until it leaves there), a transfer bias applying unit applies a predetermined transfer bias (transfer voltage) to the shaft of the transfer roller. Note that since the characteristic of the transfer roller changes due to an environmental change, temporal change, and the like, the transfer bias to be applied to the transfer roller (shaft) needs to be appropriately controlled in accordance with the characteristic of the transfer roller.
Hence, Japanese Patent Laid-Open No. 11-95581 has proposed an image forming apparatus which controls a transfer bias to be applied to the transfer roller in accordance with the characteristic of the transfer roller. Such image forming apparatus controls to set a current which flows through the transfer roller to assume a predetermined value at a timing at which a non-image-forming area is located on the transfer part (constant current control), and also controls the transfer bias on an image-forming area based on a voltage applied at this time (constant voltage control). Note that this non-image-forming area includes areas, on which no image is formed, on the front side of the leading edge of an image for one page and on the rear side of the trailing edge of that image on the photosensitive member or intermediate transfer belt. Also, another image forming apparatus has been proposed. In this apparatus, impedances of the transfer roller are computed by applying a plurality of different voltages while one non-image-forming area is located on the transfer part, a voltage, at which a current that flows through the transfer roller assumes a predetermined value, is computed, and that voltage is used as the transfer bias for an image-forming area.
On the other hand, as the primary charger, a charging member of a contact charging type represented by a charging roller becomes mainstream. The charging roller is brought into contact with the surface of the photosensitive member to apply a charging bias (e.g., a charging voltage generated by superposing an AC voltage on a DC voltage), thereby charging the surface of the photosensitive member. In this case, by setting the AC voltage to be equal to or higher than a discharge start voltage, an effect of uniforming charges on the photosensitive member is provided, thus uniformly charging the photosensitive member.
However, when a DC voltage and AC voltages are superposed and applied to the photosensitive member, since the discharging amount to the photosensitive member increases compared to a case in which only a DC voltage is applied to the photosensitive member, degradation (scraping, etc.) of the photosensitive member is promoted, and an image blur or the like due to discharge products occurs in a high-temperature, high-humidity environment. Therefore, an AC voltage to be superposed on a DC voltage needs to be minimized to suppress discharging. However, the relationship between the voltage to be applied to the charging roller and the discharging amount is not always constant, and the discharging amount changes due to an environmental change, temporal change of the photosensitive member, and the like.
To solve this problem, Japanese Patent Laid-Open No. 2001-201920 has proposed an image forming apparatus which suppresses an increase/decrease in discharging amount due to an environmental change, temporal change, and the like by controlling a charging bias to be applied to the charging roller. This image forming apparatus computes the impedances of the charging roller and discharging amounts by applying a plurality of different AC voltages for a non-discharging area and discharging area prior to image formation. During image formation, the apparatus applies an AC voltage of one value of the non-discharging area on a non-image-forming area, and determines a charging bias based on a current that flows through the charging roller at that time, and the impedances of the charging roller and discharging amounts computed before image formation.
However, an image forming apparatus disclosed in Japanese Patent Laid-Open No. 11-95581 executes constant current control of a circuit that generates a transfer bias while a non-image-forming area is located on a transfer part, and executes constant voltage control while an image-forming area is located on the transfer part. For this reason, this apparatus must include both a constant current control circuit and constant voltage control circuit. Therefore, the cost of the image forming apparatus increases.
As described above, upon computing the impedance of the transfer roller, since one non-image-forming area is located on the transfer part for a very short period of time, a transfer bias applying circuit that can change voltage values to be applied to the transfer roller at high speed is required. Therefore, the image forming apparatus needs to equip a high-voltage power supply with quick response, resulting in an increase in cost of the image forming apparatus.
On the other hand, since the image forming apparatus disclosed in Japanese Patent Laid-Open No. 2001-201920 determines a voltage to be applied on the image-forming area based on the impedance of the charging roller and discharging amount, which are predicted by applying the AC voltage of only one value on the non-image-forming area, it is very difficult to control the charging bias with high precision.